This invention relates to the field of polymers, particularly urethane rubbers and foams, and to a method of making such polymers electrically conductive or electrically semi-conductive.
In the prior art, polymers, particularly urethane rubber, have been used for a variety of applications in which it is desirable that the product have some electrical conductivity, either throughout the product itself or at least on the exposed surface of the product.
One example involves rollers used in many printers to help transport paper or carry toner in electrograhic printing. The rollers are often made of a polymer such as polyurethane or covered with a similar polymer to facilitate their paper-carrying or toner transfer ability. Different materials, including rubber, may be used in place of polymers for these applications, but because these polymers are much more durable than rubber, they are preferred. Most polymers do not conduct electricity, however, and static charges, which adversely effect the operation of the printer, can build up on the rollers. A similar problem exists if rollers of the same material are used for other purposes, such as carrying semiconductors as part of a semiconductor manufacturing process. Other end uses require conductive or semi-conductive parts as well.
As a result, there have been attempts make such polymer parts electrically conductive. In some cases, the part made from the polymer has been coated with an electrically conductive material. Unfortunately, these coatings have short life spans, and some are toxic. Another approach has been to disperse an electrically-conductive material in the polymer when the part is being fabricated. These electrically-conductive materials have included metal powders such as silver, copper, and nickel, and also materials such as carbon black, graphite, or other conductive polymers. However, the resulting products have several serious drawbacks. In the prior art, in order to make the polymer even semi-conductive, a large amount of conductive filler, e.g., metal powder like carbon black, had to be used, often as high as 10% to 40% of the overall mixture by weight. This degraded the mechanical and thermal properties of the resulting polymer part. Moreover, because of the size, and nature of the conductive particles, as well as the way in which the particles were mixed with the polymer, conductivity was not very great.
Another related problem is that it is very difficult, due to the relative size and weight of the added particles and the difficulty in dispersing them into the polymeric composition, to achieve a uniform distribution of the conductive material throughout the polymer. As a result of an uneven distribution, the electrical conductivity of the resulting product is not uniform, and the resulting product""s mechanical and thermal properties suffer as well. As a result of all this, in general, products made from such compounds have been far less than satisfactory, and in fact, in some applications, become high maintenance items.
Finally, a related problem is that it is often desirable to select the specific conductivity of an polymer in advance, as different end applications preferentially require parts with different conductivities. Selection was not really possible with the prior art methods of making semi-conductive polymers.
Accordingly, one object of the invention is to provide a method by which polymeric materials may be made electrically conductive, without the need for conductive coatings or large amounts of conductive fillers.
Another object of the invention is to provide a method of making an electrically conductive polymeric material so that the resulting product has uniform electrical conductivity throughout.
Another object of the invention is to provide a method of making an electrically conductive polymeric material so that the relative electrical resistance of the resulting product may be varied with a high degree of accuracy.
Another object of the invention is to provide a method of making an electrically-conductive polymeric material whereby the mechanical and thermal properties of the resulting product are not degraded from what would be expected with a similar material which was non-conductive.
Another object of the invention is to provide a polymeric material which is electrically conductive and which may be molded and machined.
Another object of the invention is to provide an electrically-conductive polymeric material that is free of voids.
Another object of the invention is to provide an electrically-conductive polymeric material that when used on the surface of rollers, e.g., in a printer, inhibits build- up a static charge on the roller.
Another object of the invention is to provide a polymeric material with good thermal stability.
The invention features an electrically conductive or semi-conductive polymeric material that has a resistivity of between 1012 ohm-cm and 105 ohm-cm. The material is a solid solution of metal salt dissolved in a polymer. The metal salt is complexed with the polymer, which is what provides the material with its conductive properties. Depending on the desired resistivity, the quantity of metal salt in the material can be varied, although preferably the material includes only a small amount (less than 1%, more preferably less than 0.1%) of the metal salt by weight. Because only a small amount of the metal salt is included in the material, the material has good mechanical and thermal properties. These properties, coupled with the conductive nature of the material, make the material suitable for use as coatings on, e.g., rollers used on paper printers.
The preferred polymers contain, e.g., nitrogen, oxygen, sulfur or halide atoms, or unsaturated (double and triple bond) groups, which are available for complexing with the metal salt. Preferred polymers include elastomeric polymers like polyurethanes and rubbers, adhesive polymers, and plastics. When rubbers are used, the material also preferably includes a plasticizer.
Preferred metal salts suitable for use in the material include transition metal halides like CuCl2, CuBr2, CoCl2, ZnCl2, NiCl2, FeCl2, FeBr2, FeBr3, CuI2, FeCl3, FeI3, and FeI2. Other preferred salts include Cu(NO3)2, copper lactate, copper tartrate, iron phosphate, iron oxalate, LiBF4, and H4Fe(CN)6.
The invention also features a method of preparing these polymeric materials. Generally, the method includes making a homogenous solution of the metal salt in a polymer or polymer precursor, and curing the composition. When the polymer precursor is an isocyanate functional prepolymer, the solution also includes an extender (polyol or polyamine) that reacts with the isocyanate groups during curing to form a polyurethane resin. This method results in an even distribution of the metal salt throughout the polymeric material, which provides the material with uniform conductivity throughout.
The conductive elastomers are suitable for use in a variety of industrial applications to control surface charge and to provide good heat conductivity and expanded life. For example, the polymers can be used to coat the belts, shafts, wheels, inserters, and paper handling and copier toner pick-up rollers in paper printers. The polymer can be used to coat car bodies, print circuits, seals, and to dissipate charges in various other electrical applications, such as coating on belts that are used to transport semiconductor wafers during manufacture. The conductive plastic materials (such as nylon) can be used to coat disc drives, machine body parts, cabinets, and carry cases.
Other features and advantages of the invention will be apparent from the description of the preferred embodiment thereof, and from the claims.
The preferred polymers are polyurethanes. The preferred method of preparing conductive polymeric materials including a polyurethane is to mix an extender (polyol or amine) or an isocyanate-functional prepolymer with a solution of a metal salt. The mixture is then cured. Of course, other standard ingredients, like a cure accelerator or a flame retardant, may be included in the mixture.